In the plastic injection molding art, the usual challenges facing a product designer is to design an article having requisite strength for the product application and uniform surface quality for satisfactory appearance, but to avoid excessive weight, material usage and cycle time. A design compromise must often be made between strength and plastic thickness. A relatively thicker plastic section in the article, such as a structural rib, will incur greater weight, material usage, cycle time and induce sink marks and other surface defects due to thermal gradients in the area of the thickened section.
It is known in the plastic molding art to use pressurized fluid in conjunction with the plastic molding of articles as shown in U.S. Pat. No. 5,098,637 to Hendry. The pressurized fluid is typically nitrogen gas which is introduced into the mold cavity at or near the completion of the plastic injection. The pressurized fluid serves several purposes. First, it allows the article so formed to have hollow interior portions which correspond to weight and material savings. Second, the pressurized fluid within the mold cavity applies outward pressure to force the plastic against the mold surfaces while the article solidifies. Third, the cycle time is reduced as the gas migrates through the most fluent inner volume of the plastic and replaces the plastic in these areas which would otherwise require an extended cooling cycle. Fourth, the gas pressure pushes the plastic against the mold surfaces, thereby obtaining the maximum coolant effect from the mold.
However, as the dimensions of the molded article increase, the gas must do more work to migrate through the volume of the mold cavity to assist in setting up the article within the cavity. If the pressure of the gas is too great as it enters the mold cavity, there is a risk that it may rupture or blow out the plastic within the mold cavity, i.e., the gas is not contained within the plastic. Thus, there have been practical limitations in the adaptation of gas injection in the plastic molding field.
The above-noted U.S. patent to Hendry solves most of these problems.
As illustrated in FIGS. 1-4, another approach is to use a fixed volume spill cavity 24 with a block pin 20 and shims 26 to control the volume of plastic going into the spill cavity 24.
In FIG. 1, the pin 20 is in its up position blocking plastic flow from the molding in the article-defining cavity 12, through a runner 23 and into the spill cavity 24. In FIGS. 2-4, the pin 20 is in its down position allowing plastic to go to the spill cavity 24 by the pressure of the gas.
However, injection molding machines do not deliver the same quantity of plastic shot after shot. When doing straight compact injection molding, the cushion of plastic in front of the screw after the mold is full and the high pressure packing starts, takes care of this inaccuracy of the screw ram to deliver the same quantity of plastic on each shot (i.e., the cushion can fall from 10 mm to 5 mm with no difficulty in molding the compact injection molding).
However, with some parts, one does not want to pack the molding. One wants to just fill the cavity. If one packs the molding, a strain pattern will already be in place within the molding and the hollowing out of the part will not relieve the strain already in place in the molding.
FIG. 2 illustrates blow out in the spill cavity 24 causing a loss of gas pressure in the molding in the article defining cavity 21. One result of this is a possible shrinkage due to loss of gas pressure in the molding in the article defining cavity 21. Also, gas leakage may occur at the mold parting line.
FIG. 3 illustrates the use of too much plastic which would result in sink marks at reference numeral 26.
Finally, FIG. 4 illustrates the addition of the shims 26. However, overpacking and blow out as illustrated in FIGS. 3 and 2, respectively, can still occur.